The importance of automated mechanical transmissions will increase in the future. Compared to manually operated transmission, they offer a considerable increase in travel comfort and travel security at insignificant higher costs, inasmuch as they unburden the driver, but compared to conventional automatic transmissions they have clearly lower production costs and reduce small fuel consumption.
Important for the operation of automated mechanical transmissions is the automatic control of clutch actuation and the knowledge, necessary for this purpose, regarding the application point of the clutch in all operating conditions, such as elevated temperatures, deteriorated clutch linings, etc. The application point is designated as the position of the clutch at which the transmission of torque from the engine to the transmission input shaft, begins.
In automated mechanical transmissions, the driver or a control device for shifting the transmission pre-selects the next gear whereupon the specific shifting operation automatically start. To keep the whole shifting time as short as possible, a pure dead time in which the clutch approaches the application point, i.e., to carry out this approach as quickly as possible, the motion of the clutch that follows this in an engaging direction progresses under control and in a manner such that the shifting operation can be carried out at a shifting rotational speed of the transmission input shaft.
Already known from EP 0 392 762 B1 is a method for determining the application point of an automatically actuated friction clutch of a mechanical transmission for a motor vehicle in which the idling rotational speed of the engine is monitored and used for determining the application point. With the first engagement of the clutch, the engine rotational speed slows down as consequence of the resistance from starting a stationary transmission. In this significant drop of the idling rotational speed of the engine at the start of the clutch engagement, conclusion is reached as to the application point. It is disadvantageous for detecting the application point with certainty that a relatively great torque has to appear. In the engine this leads, as a rule, to an interruption in rotational speed that the driver feels as disturbing. Another disadvantage of the known method is seen in that it can only be used together with a transmission having a conventional synchronization device at its disposal, since synchronization, via a rotational speed control of the engine, does not result.
WO 98/28162 A1 or EP 0 883 509 B1 discloses a method in which the engine rotational speed is likewise monitored and used for determining the application point of a clutch. This known method uses a downshift operation. At the same time, after disengaging of the clutch and disengagement of the higher gear by engaging the lower gear, the transmission input shaft is brought to a higher rotational speed. When disengaging the clutch, the engine is consequentially accelerated to the higher rotational speed. During this important rotational speed increase, conclusion is reached as to the application point. This method also can only be used in combination with a transmission having conventional synchronization device, since synchronization does not result by an active rotational speed control of the engine.
With this background the problem on which the invention is based is to create a method, which allows to reliably and accurately to determine the application point of the clutch and which makes an active synchronization possible by control of the rotational speed of the engine.